The present invention relates, in general, to a method and apparatus for locating the borehole of a deep well at a substantial distance below the earth's surface, and more particularly, to a method and apparatus for locating such a well bore from a relief well which is approaching the target well bore at a large angle of intersection.
It is well known that in drilling deep wells for oil and gas exploration, precise control of the path followed by the well is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of plus or minus one-quarter of a degree will allow the bottom of a 10,000 foot well to be positioned anywhere within a circle 100 feet in diameter, and numerous factors can increase this deviation. This is not of particular concern in normal drilling operations, but if an accident should occur, such as a rupture in the casing of the well which causes the well to blow out, it may become necessary to plug the well at or below the point of the rupture to bring it under control. In order to do this, a relief well must be drilled to intersect the original well at the desired level. Since such blow-outs produce extremely hazardous conditions at the earth's surface in the vicinity of the original well head, the relief well must be started a considerable distance away and drilled at an incline in order to protect the personnel and the equipment used.
Because the same problems of control of the direction of drilling are ncountered in the relief well as were encountered in the original well, the location of the relief well bore cannot always be determined with precision; accordingly, it is extremely difficult to determine the distance and direction from the relief well to the blown out well with sufficient accuracy to insure that the relief well will intersect with the target well. A further difficulty is that in some cases the force of the explosion resulting from the rupturing of a well casing can do so much damage at the surface that it may be difficult to say exactly where the well head is, much less where the well is located at the depths of 10,000 feet or more. In addition, the relief well often is started one-half mile or more away from the area of the original well head, because of the conditions there, and is angled toward the original well until it approaches relatively closely. Then the relief well may be caused to angle downwardly to follow along the original well to the location where the two wells are to intersect. Thus, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to the original well.
A further difficulty is encountered when it is desired to drill the relief well so that it intersects with the target well at a large angle of intersection. This occurs, for example, when the point of intersection at the target well is to be relatively shallow so that the relief well must approach it in a generally horizontal direction. In such cases, the relief well approaches the target well very quickly, and can easily by-pass the target if an accurate measurement of direction is not obtained, or can intersect it unexpectedly if an accurate measurement of distance is not available.
Numerous attempts have been made to provide accurate measurements of well location, and many surveying techniques are available which can give general information as to the location of a relief well with respect to a target well. However, such survey techniques are not capable of providing accurate data concerning the relationship of the relief well to the original, or target well, when there is a high angle of approach.
The art of electrical well logging is highly developed, and various systems have been successfully used in the past in locating geologic anomalies of certain types, but these systems have been less than satisfactory in guiding relief well bores to locate and intersect existing boreholes. Many of the difficulties encountered in such prior systems were overcome by the methods and apparatus disclosed in U.S Pat. Nos. 4,323,949 and 4,372,398 of Arthur F. Kuckes, the disclosures of which are incorporated herein by reference. In accordance with the '398 patent, an alternating electric current flow is produced in the casing of a target well, or borehole, by injecting a current into the earth surrounding the target well. Preferably, an electrode located in the relief borehole is used, this electrode being carried by the logging cable and contacting the uncased sides of the relief borehole so that a current flow is established in the vicinity of the relief hole. This current flow extends between the downhole electrode and a second electrode which, in a preferred form of the invention, is located at the earth's surface in the vicinity of the head of the relief well. The relief well is uncased, so the injected ground current finds a path of least resistance through the casing or other current-conducting material in the target borehole, and the resulting concentration of current produces an alternating magnetic field surrounding the target well which can be detected by an A.C. field sensor mounted in a sensor tool, as described in the '848 patent. The sensor disclosed in the '848 patent is extremely sensitive to very small magnetic fields, and permits an accurate detection of currents flowing in borehole casings located a considerable distance away from the relief borehole. The current generated in the target borehole casing is an alternating current of relatively low frequency which produces a characteristic magnetic field that can be detected and which produces a corresponding output signal from the A.C. magnetic field sensor. The signal produced by the A.C. magnetic field sensor provides a measure of the direction and distance to the target well casing.
In the course of drilling a relief well, A. C. magnetic field measurements are taken periodically at selected depths, and the resulting measurements, when taken with simultaneous measurements of the earth's static magnetic field to provide a stable reference point for orientation of the A.C. field sensor, will, in accordance with U.S. Pat. No. 4,372,398, permit periodic calculations of the presumed location of the target well with respect to the location of the A.C. magnetic field sensor in the relief well. This information is used to guide further drilling of the relief well. As the relief well approaches a desired depth, its approach to the location of the target well can be guided by the output of the sensor tool so that the target well is intersected at the desired depth below the earth's surface in a rapid and effective manner, without ambiguity, and with resulting savings in the cost of drilling the relief well.
Although the method and apparatus set forth in the '398 and '848 patents were found to be extremely effective, it was found that the need to detect and measure the earth's magnetic field in order to establish the location and orientation of the sensor within the relief well introduced errors, in some situations, for when the earth's magnetic field becomes distorted or difficult to measure, the results became unreliable. More specifically, in accordance with the '398 patent, a measurement of the two vector components of the earth's magnetic field lying in the plane perpendicular to the axis of the sensor was required in order to obtain a complete specification of the sensor orientation in space so as to be able to determine the precise direction of the target well from the data received by the A.C. magnetic field sensor. It was found, however, that this method of orientation was imprecise in situations where the borehole axis nearly coincided with the earth's magnetic field lines and further, if the earth's magnetic field was perturbed by ferromagnetic effects around the relief or target well, the determination of sensor orientation could be faulty.
The foregoing difficulties were overcome in accordance with the invention set forth in U.S. Pat. No. 4,700,142, of Arthur F. Kuckes, wherein sensor orientation is obtained by making measurements of the inclination and direction of the relief well from conventional borehole survey data; then the rotational orientation of the A. C. field sensor tool within the relief well is determined by means of a gravity measurement obtained, for example, by means of vector accelerometers. Taken with the data concerning the slope and direction of the relief borehole, the accelerometer output permits a precise determination of the orientation of the sensor tool and thus permits a determination of the direction and distance of the target from the relief well sensor, without the need to measure the earth's magnetic field.
However, if the relief well approaches the target well at a large angle, for example, between 60.degree. and 90.degree. of the axis of the target well, it becomes very difficult to obtain meaningful and accurate data, since the A.C. sensors, which are perpendicular to the axis of the sensor tool, then will lie in a plane which approaches being parallel to the axis of the target well. Since the essential A.C. signal information lies in a direction which approaches being perpendicular to the sensor plane, or parallel to the axis of the relief well borehole, it becomes very difficult to produce the required analysis of the received data and to determine the direction of the target well with accuracy and precision. Thus, an improved apparatus and method for providing an accurate and reliable measurement of the direction and distance to a target well from a relief well approaching at a high angle of intersection is needed.